The present invention relates to a cutter head for a suction dredger provided with a row of blades extending substantially in a cylindrical surface, which row of blades is rotatable about a central tilting shaft.
Such a cutter head for a free-sailing trailing dredger is described in Dutch Pat. No. 124,103, wherein the blades are mounted at right angles to a superimposed carrier ring and a pair of such cutter heads, each having its own driving gear, is secured to the arm pivoting up and down relative to the vessel. By varying the rotation speed and rotation direction of the two cutter heads, there may be achieved an orienting effect on the vessel and hauling ropes or the like.
Cutter heads controlling means may be omitted. The soil loosened by the cutter heads is discharged as a mixture with water by means of a dredge pump and via a suction conduit, the inlet nozzle of which lies at the level of the carrier ring.
Through the tiltability of the axis of rotation of the row of blades, it is possible with such cutter heads, irrespective of the angle of inclination of the ladder, that the row of blades may be fixed in any required position, e.g. horizontally, so with vertical axis of rotation, in which case a horizontal bottom may be delivered.
A drawback going with such prior art, disc-shaped cutter heads, indeed all prior art rotary cutter heads, even that of the crown type, is the inadequate formation of a properly suctionable mixture of the loosened soil and the transport water. As a result thereof spillage occurs, the definition of spillage being soil that is cut loose but which does not arrive in the suction tube. The mixture formation in prior art cutter heads is inadequate, since the transport of the loosened material to the suction nozzle, i.e. the inlet of the suction line, has to be done exclusively by the under-pressure in the suction nozzle and this rapidly decreases with increasing distance to the suction nozzle. It is clear that upon increasing dimensions of the cutter heads and consequently increasing distance from the loosening place to the suction nozzle, likewise the quantity of spillage increases.
It is the object of the present invention to avoid this drawback.
To this effect according to the invention there is disposed a closed bottom plate at the lower end of the blades, a suction nozzle being arranged in scooping relationship within the space defined by the row of blades, which means that the face of the suction nozzle lies approximately in a radial plane and the bottom plate being rotatable in the direction of the suction nozzle. Radial plane is intended to mean a plane which extends in a radial direction and includes the axis of rotation of the cutter head in that plane.
Soil loosened by the blades in the suction head according to the invention is brought within the cutter head through the bottom plate in the direction of the suction nozzle, so the transport of the mixture to the suction nozzle no longer depends exclusively on the under-pressure in the suction nozzle. By the specific arrangement of the suction nozzle, moreover the transport distance can be minimized, while the loosened soil need no longer be conducted upwardly to the suction nozzle.
Preferably the blades are secured to the bottom plate and are driven via the bottom plate. As a result a stationary suction nozzle may be arranged within the rotating row of blades in a constructively simple manner.
In a cutter dredger of the type wherein a ladder with a cutter head at the lower end is brought in a swinging movement by means of hauling winches and after each swing the vessel is hauled, so that the cutter head executes substantially concentric dredging cuts, excavation will take place alternately "concurrently" and "counter-currently" upon rotation of the row of blades in one direction, depending on the swinging direction of the ladder. By counter-current, counter-rotating or counter-cutting suction dredging is meant that, seen in top view, the rotation direction of the row of blades at the front side is identically directed to the swinging direction of the ladder. In case of concurrent suction dredging, these directions are opposite.
Since in case of concurrent suction dredging the soil is loosened in another zone of the circumference of the cutter head than in case of counter-current suction dredging, in case of a stationary suction nozzle, the transport distance of the loosened soil as far as the suction nozzle will be different in these two cases.
Consequently, in a preferred embodiment of the invention, the cutter head may be fitted with an attachment in the form of a bent tube disposed concentrically within the row of blades, and which is open on either side and pivotable between a position wherein one tube end adjoins the suction nozzle and a position spaced apart from the suction nozzle.
The attachment therefore constitutes as it were an adaptor or an extension piece of the suction tube with which the suction nozzle can be brought adjacent the place where the soil is loosened at a given swinging direction of the ladder, while the position of the stationary suction nozzle is so chosen that this, during the swinging of the ladder, is present in the other direction, directly downstream of the soil loosening zone.
Thus, a very short suction path can be guaranteed under all circumstances, which path moreover is only horizontal and whereby in conjunction with the closed bottom plate, very high soil concentrations in the suction mixture are possible.
For example in thick mud the tubular adaptor avoids loosened material to be uncontrollably diluted by inflowing water which would adversely affect suction efficiency.
Instead of a curved tubular and pivotable adaptor, a shield extending along the inner side of the row of blades, which shield is pivotable between a position with one end adjacent the suction nozzle opening and a position spaced thereof, can have the same effect. In the position of the shield adjacent the suction nozzle, the material will move through an annular space defined by said shield, the rotary bottom plate, the upper wall and the central sleeve.
The adaptors referred to above, whether they are of the curved tubular type or of the curved shield type are displaceable to and from the suction nozzle opening which is stationary within the row of blades and the adaptors have the effect of displacing the effective suction nozzle opening in such a way that said opening is always positioned close to the area where soil material is loosened by the row of blades.
It is however also possible to position the suction nozzle opening in different areas within the suction head without making use of any adaptors or other auxiliary means, but by moving the suction nozzle itself. Because the suction nozzle must communicate on an upwardly transporting suction line, said suction line must be branched above the suction head and be connected thereto in two different places, wherein the suction nozzle is moveable from each one of said connections to the other. To prevent that when the suction nozzle is positioned under one of said connections, water is sucked in through the other connection, which obviously is undesirable. According to the invention the suction nozzle is mounted on a plate, which apart from a passage for loosened soil through the suction nozzle to the relevant suction line branch, is fully closed. The plate is pivotable together with the suction nozzle and is arranged below the upper wall of the cutter head. Accordingly whenever the suction nozzle is under one of the two suction line connections the other is automatically closed off.